Method and apparatus for monitoring the wear on a rotary drill bit

ABSTRACT

A method and apparatus for monitoring the wear on a rotary drill bit including means for measuring the revolutions of the drill string and the weight on the bit, and for obtaining the product thereof, such product being a measure of the wear on the bit. A pulse train is generated having a pulse rate proportional to the weight on the bit. Controlled portions of the pulse train are registered in a counter, the portions being proportional in duration to the number of revolutions turned by the bit, with the result that the accumulated count registered provides at any time the integrated product of weight and revolutions, as a measure of bit wear.

United States Patent Rundell et al.

[ NOV. 27, 1973 METHOD AND APPARATUS FOR MONITORING THE WEAR ON A ROTARYDRILL BIT [75] H Inventors: Herbert A. Rundell; James lj D avis,

"samarnaamjrif [73] Assignee: Texaco Inc.,New York, NY. zzfriledl Nov24, 1 971 [21] Appl. No.: 201,651

[52] US. Cl. 73/151, 175/39 [5]] Int. Cl E21b 47/00 [58] Field of Search73/151, 151.5, 133; 175/39 [56] References Cited UNITED STATES PATENTS2,365,014 12/1944 Silverman et al. 73/l51.5

3,581,564 6/1971 Young, Jr. 73/151 3,541,852 11/1970 Brown et al. 73/151I Primary Examiner-Jerry W. Myracle Att0mey-Thomas H. Whaley et al.

[5 7] ABSTRACT A method and apparatus for monitoring the wear on arotary drill bit including means for measuring the revolutions of thedrill string and the weight on the bit, and for obtaining the productthereof, such product being a measure of the wear on the bit. A pulsetrain is generated having a pulse rate proportional to the weight on thebit. Controlled portions of the pulse train are registered in a counter,the portions being proportional in duration to the number of revolutionsturned by the bit, with the result that the accumulated count registeredprovides at any time the integrated product of weight and revolutions,as a measure of bit wear.

9 Claims, 3 Drawing Figures 613m; gas 554m? TOT/M/Z/NL; cows r52 r '1 mg/5722 K/L OPOU/VDS mum/1.

METHOD AND APPARATUS FOR MONITORING THE WEAR ON A ROTARY DRHJL BITBACKGROUND OF THE INVENTION In rotary drilling operations, such asdrilling for oil, a drill bit is lowered on the end of a drill pipe tothe bottom of the borehole. The weight of the whole string of drillpipe, including one or more heavy drill collars just above the bit, islarge, usually measured in thousands of pounds, or kilopounds. Thisweight is sensed on the drilling platform as the hook load on thetraveling block of the drilling rig. When the bit is resting or drillingon bottom, some of the total weight of the drill string is borne by theearth formation under the drill bit, while the remainder is still borneby the hook and thus appears as a reduced hook load on the drilling rig.The amount by which the hook load' is reduced when the bit is on bottomrepresents the downward force or thrust of the drill bit upon the earthformation. This difference in hook load is called the weight on the bit,and is measured customarily in kilopounds.

After a drill bit has been in operation for an extended time it becomesworn, with the result that optimum rates of drilling are no longerachieved. Continued operation under these conditions is costly.Moreover, continued use of a worn bit may lead to the failure of thebit, with the possibility of parts of the bit coming loose in theborehole and presenting an obstacle to subsequent drilling.Consequently, it is of interest to the drilling operator to monitor thewear to which a drill bit has been subjected during its use in aborehole, in order to make the most efficient use of the drillingequipment.

The amount of wear on a drill bit may be measured by the product of theweight on the bit, which may be expressed in kilopounds, and the amountof rotary (or angular) movement, in revolutions, that the bit hasexperienced, or to be more precise, it can be measured by the product ofthe weight on the bit during a certain time interval and the amount ofrotary movement that the bit makes during that same time interval, thendetermining the same product for a succeeding time interval, and againfor a still later time interval, and so on, and then summing all suchproducts over all such time intervals in order to obtain the kilopoundsrevolutions product for the entire time that the bit is turning onbottom. The present invention comprises a method and apparatus forcontinually making measurements at the drilling platform and derivingfrom these measurements 3. quantity that is a measure of the wear on thedrill bit.

It is recognized that there have been proposed various ways to generateand record analog signals which are indicative of the weight on the bit,the revolutions that the bit has experienced as well as the product ofthese quantities. However, the present invention pro vides a superiormethod and apparatus for accomplishing such objectives in a manner thatis particularly well adapted to deriving its inputs from knowndrilling-rig equipment. In addition, this invention is more accuratethan any known prior system. The latter is because of the fact that thepresent invention provides for taking the sum of incremental productsrather than the simple product of any particular value of the weighttimes the revolutions.

SUMMARY OF THE INVENTION According to the present invention, a measureof the product of weight and rotary (or angular) movement is obtained asfollows. For every increment of preselected magnitude of rotary movementof the drill bit one gate signal of preselected time duration isinitiated. At the same time, there is continuously generated a pulsetrain having a pulse rate proportional to the weight on the bit. Thepulse train is continuously introduced into a counter for pulses, saidcounter being enabled to register such pulses only during the timeinterval while an external gate signal is being applied to the counter,or in other words, only while such gate signal is maintained at an inputto the counter. Each of the aforesaid gate signals of preselected timeduration is applied as an external gate signal to the counter, thusenabling the counter to register all pulses which occur within the timeduration of each such gate signal. The number of pulses thus registeredby the counter during the application of each gate signal is a digitalnumber whose magnitude is a measure, i.e., a digital display, of theproduct of the weight on the bit and one increment of rotary movement ofthe bit. Such product is a measure of the wear on the bit during eachsuch increment of rotary movement.

Finally, the total number of pulses registered by the counter during theapplication of all such gate signals is a digital display of the sum ofthe products of the weight on the bit and the amount of rotary movementthat the bits makes with each such weight on the bit. The drill-bit wearis measured by such sum of the products and, therefore, also by thetotal number of pulses registered by the counter.

The increments of rotary movement of the drill bit are always taken ofthe same preselected magnitude, and correspondingly the signalsinitiated for each such increment are also always of the samepreselected time duration. If the pulses of the pulse train that occurduringall the successive increments of rotary movement of the bit arecounted or totalized, the total will be a digital number proportional tothe sum of the products of the weight on the bit and each correspondingincrement of rotary movement of the bit. Since the weight on the bit iscustomarily measured in kilopounds and the rotary movement of the bit ismeasured in revolutions, the total count so obtained for all operationswith a given bit will be proportional to, i.e., will be a digitaldisplay of, the summed or integrated product of the weight on the bitand the rotary movement of the bit for all increments of such rotarymovement. In other words, the total count will be a digital display ofthe kilopounds-revolutions to which the bit has been subjected.

It is a principal object of the present invention to provide a methodand apparatus for continually monitoring the wear on the drill bit.

It is another object of this invention to provide a method and apparatusfor continually making measurements at the surface of the earth andcontinually deriving from these measurements a quantity that is ameasure of the wear on the drill bit, so that the operator will be ableto monitor the wear on the bit and to estimate at any time the amount ofuseful life remaining in the bit.

It is still another object of this invention to provide a method andapparatus for continually making measurements at the surface of theearth and continually deriving from these measurements a digitalquantity that is a highly accurate measure of the wear on the drill bit,which digital quantity retains its high accuracy over extended periodsof time.

These and other objects, advantages and features of the invention willbe more fully set forth below in connection with the best modecontemplated of carrying out the invention as set forth in the followingdescription taken in conjunction with the accompanying drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a representation of adrilling rig and, in block-diagram form, the manner in which the methodand apparatus of this invention can be tied in with the rig to producethe desired result, namely, a continuously obtained measure of the wearon the drill bit;

FIG. 2 is a more detailed schematic circuit diagram of the portion ofFIG. 1 which is represented by block diagram; and

FIG. 3 is a time-sequence diagram of the voltage signals developed inthe system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1 of thedrawings, there is shown a schematic representation, or block diagram,of the system of the invention as it ties in with a drilling rig.Drilling line 44 passes from a draw works 45 over a crown block 46 andtraveling block 47. The latter supports the drill stern by means of atop joint of the drill stem, or a kelly 48. The anchored end of thedrilling line, or a dead line 50 is at all times under a tension whichis proportional to the line-supported weight of the drill stem, i.e.,the hook load. Such tension is continually measured by a hook-loadweight indicator 51 which is connected to the dead line 50 at thedead-line anchor.

The drill stem is driven in rotation through the action of a rotarytable 52 upon kelly 48. The rotary table 52 is driven in rotation by ashaft 53 which, in turn, is driven by a rotary drive engine 54. The rateof rotation is continually measured by an A.C. tachometer 55, which maybe an integral part of rotary drive engine 54. Also, a time signalindicating the completion of each preselected and fixed increment ofrotary (or angular) movement of the bit is revolutions (usually afractional value of one revolution) is fed from the tachometer 55 into aRevolutions Channel" at input terminal 2.

A signal proportional to the number of kilopounds weight supported bythe drilling line, i.e., the hook load, is fed from the hook-load weightindicator 51 into a Kilopounds Channel at input terminal 1. Then, afterappropriate conversion of the kilopounds signal to a pulse train havinga pulse rate proportional to the weight on the bit in the Weight ToPulse Rate Converter" 3, a totalizing counter 5 registers the pulsetrain during the time of a gate signal. Such gate signal is derived fromthe time signal, and it is a time gate signal of preselected and fixedtime duration which is developed in the Gate Generator 4. Consequently,the total number of counts showing at any time on a register 6 is ameasure of the total number of kilopoundsrevolutions to which the bithas been subjected.

In order to make the register 6 direct reading inkilopounds-revolutions, one can calibrate it either by directexperimental calibration, or by using the readily found apparatusconstants, namely, the ratio of kilopoundsto-pulse rate for converter 3and the number or fractional number of revolutions per preselectedincrement of rotary movement between successive time gates for the gategenerator 4.

While one skilled in the art might devise other equivalent means, FIG. 2illustrates in more detail some of the elements disclosed schematicallyin FIG. 1. In FIG. 2 elements which correspond to elements in FIG. 1 aredesignated by like reference characters and numbers.

The input to the kilopounds channel is derived in the following mannerfrom the reading of the hook-load weight indicator 51. Such a weightindicator consists of a pressure transformer (not shown) attached to thedead line 50 at the dead-line anchor. Tension on the dead line 50exercises a transverse force on the pressure transformer which, in turn,develops a corresponding pressure signal in an hydraulic tubing 7 whichleads to a transducer 8. Transducer 8 consists of a potentiometer 9driven by a Bourdon tube 10 and a mechanical movement 11. It acts insuch a manner that as the hook load increases, the pressure in tubing 7increases. Consequently, a moving contact 12 is moved along thepotentiometer windings, with the result that the voltage that appears onthe lead connected to contact 12 is correspondingly increased. There isa battery 13 which supplies the electromotive force to potentiometer 9.And, connected in parallel with potentiometer 9 and supplied by the samebattery 13, there is a second potentiometer 14, which is a duplicate ofpotentiometer 9. It has a moving contact 15 which is manuallyadjustable.

The foregoing hook-load weight indicator might be a Type E weightindicator which is manufactured by the Martin-Decker Corporation, SantaAna, Calif. It is described in their Bulletin P-92.

In operation, contact 15 is set at the same electromotive force valuealong potentiometer 14 as contact 12 is along potentiometer 9, while thedrill bit is off the bottom of the hole. This is done by adjustingcontact 15 until there is a null between contacts 12 and 15 while thebit is off the bottom. Then, when the drill bit is resting or drillingon bottom, the pressure in tubing 7 is found to be reduced by an amountrepresentative of the weight on the bit, and contact arm 12 assumes anew position at a lower electromotive force representative of thereduced hook-load weight. The contact arm 15 remains where it waspreviously set on potentiometer 14. The difl'erence in electromotiveforce between contacts 12 and 15 on potentiometers 9 and 14,respectively, is thus a DC. electrical signal representative of thedifference in the hook load before and after lowering the bit onto thebottom, i.e., a bias signal representative of the weight on the bit.Consequently, transducer 8 with its potentiometer 9 thus biased bypotentiometer 14 is said to be a biased weight transducer.

The DC. electrical signal so derived is introduced into a conventionalDC. to frequency converter 16 which generates an alternating output 17having a frequency proportional to the amplitude of the input signaland, thus, proportional to the weight on the bit. This alternatingsignal 17 is fed into the totalizing counter 5 where every positive (ornegative) half-cycle of the alternating signal is counted, provided thegate of counter 5 is open. Opening of the gate so as to enable counter 5is determined by the signals from the revolutions channel, as explainedbelow. Register 6 presents the total number of such half-cycles counted.It may be noted that this is the same pulse train which was previouslydescribed in connection with FIG. 1.

The input at terminal 2 of the revolutions channel is derived directlyfrom an AC. tachometer and is an alternating signal having a frequencydirectly proportional to the frequency of rotation of the rotary tableand hence also of the drill bit. Such a signal may be derivedadvantageously from the windings of a torque meter such as thatdescribed in US. Pat. No. 3,295,367, wherein, as described in thepatent, there are 30 electrical cycles generated per revolution of therotary drive shaft. For a typical case where there is a gear ratio suchthat there are five revolutions of the drive shaft for one revolution ofthe rotary table, there will be 30 X 5 or 150 electrical cyclesgenerated by the tachometer per revolution of the rotary table.

In this case, if we were to use as the time signal every positivehalf-cycle of the AC. electrical signal derived from the tachometer, thepreselected and fixed increment of rotary movement so determined wouldbe 1/150 of one revolution. Thus, referring to FIG. 3, signal A afterisolation and skaping in section 18 of the revolutions channel, yieldssignal B. Signal B is nearly a square pulse as a result of a transistor23 being driven to saturation.

However, after dividing-by-two in section 19 of the gate generator 4,signal C is obtained, where it will be seen that only every otherpositive-going pulse of signal B has been passed. It is advantageous, aswill be explained below, to use every other positive half-cycle of theA.C. signal, rather than every half-cycle. Therefore, the preselectedand fixed increment of rotary movement employed for generating the timegate is 1/75 of one revolution instead of 1/150 of one revolution.

With reference to FIG. 2, signal C is pulse-shaped in a section 20 ofthe gate generator 4, and it yields signal D with its pulse shaped as toboth height and width. These pulses D are used to trigger a gatingsection 21, wherein each input pulse of signal D is caused to initiate agate pulse of preselected and fixed time duration. In the exampleillustrated, this is one of the pulses E which each have a duration of 5milliseconds. Each of these gate pulses terminates after such timeduration, and the next gate pulse is not initiated until the arrival ofthe next triggering pulse of signal D. These gate pulses of signal E arefurther amplified in a gateamplifying section 22 to provide gate signalsF which have sufficient amplitude to operate totalizing counter 5 whilealso having the same preselected and fixed time duration as did signalE.

Pulse-shaping section 18 contains N-P-N transistor 23 which is connectedin common-emitter circuit configuration. It has its emitter grounded,its collector supplied from a plus 20-volt B-supply through a resistor24, and its base connected to ground through a resistor 25. Signal A isintroduced into this pulse-shaping section 18 through a resistor 26 anda capacitor 27 connected in series between input terminal 2 and the baseof transistor 23.

Section 19 contains a divide-by-two operational amplifier 28 which hasits input supplied through a resistor 29 from the collector of.transistor 23, and its output supplied through a resistor 30 to theinput of the pulseshaping section 20. This divide-by-two amplifier 28may be an integrated circuit which is wired as a flipflop and an ANDgate. As will be explained below, the dividing by two is necessary inthe particular embodiment of the invention in order to allow a gate timesufficiently long such that at high rotary speeds and light bit loads itwill permit the counter 5 to count a statistically significant number ofhalf-cycles of signal 17 (from the DC to frequency converter 16). It maybe noted that a satisfactory converter might be one such as the AnadexModel DF-l 10R unit, having 10 to kilohertz full scale, which ismanufactured by Anadex Instruments, Inc. of Van Nuys, Calif.

Pulse-shaping section 20 consists of N-P-N transistors 31 and 32, bothin common-emitter circuit configuration, having their emitters grounded.The input to section 20, taken through resistor 30 from section 19, isapplied to the base of transistor 31, the collector of which is suppliedby a plus 3-volt B-supply through a resistor 33. Transistor 32 iscoupled to transistor 31 through a resistor 34 which has one endconnected to the collector of the transistor 31 and the other endconnected to one terminal of a capacitor 35. Capacitor 35, in turn, hasits other terminal connected to the base of transistor 32. The base oftransistor 32 is also connected through resistor 36 to ground. Thecollector of transistor 32 is supplied by a plus 3-volt B-supply througha resistor 37.

The output from pulse-shaping section 20 is taken from the collector oftransistor 32, and it is directcoupled to the input of a one-shotoperational amplifier 38 which is in gating section 21. This operationalamplifier 38 is an amplifier which produces an output gate pulse ofpreselected voltage level and time duration whenever it receives atrigger pulse at its input. Suitable amplifiers for this purpose arecommercially available in integrated circuit form.

The time duration of the output gate pulse is determined by a capacitor39 which is connected to appropriate terminals provided for such purposeon the one-shot operational amplifier. Gating section 21 consists of theoperational amplifier 38 and a resistor 40.

The output of operational amplifier 38 is connected through the resistor40 to the input of the gateamplifying section 22, at the base of atransistor 41. Transistor 41 is connected in common-emitterconfiguration, with its emitter grounded and its collector sup pliedfrom a plus 5-volt B-supply through a resistor 42. The output fromgate-amplifying section 22 is taken from the collector of transistor 41and is fed directly into the totalizing counter 5.

A suitable counter for use as a totalizing counter might be oneavailable commercially which is designated as the Anadex Model CF-604RPreset Scaler Timer. This would be operated in BATCH mode, asrecommended by the manufacturers, in order to get read-out in thedesired engineering units. The indicated counter is made by AnadexInstruments, Inc., Van Nuys, Calif.

In operation, when a new drill bit is placed in use, and while the bitis just off the bottom of the hole, and the mud pumps are running, theoperator would zero the biased weight transducer 8 (FIG. 2) by manuallysetting the sliding contact 15 of potentiometer 14 (in the kilopoundschannel) to produce a null between contacts 12 and 15. Such null wouldbe observed by reading a volt-meter 43 which is connected across theinput to the converter 16. This null condition assures that contact 15is correctly set for applying a voltage level corresponding to zeroweight on the bit to the converter 16.

Then, the bit would be lowered to the bottom of the hole, andthereafter, while drilling is under way, contact 12 would automaticallyand continuously be adjusted to positions on potentiometer 9, havingreduced voltages corresponding to lower values of hook load, as alreadyindicated above. Consequently, the D.C.- voltage difference betweencontacts 12 and 15 (which is the output of the biased weight transducer)would be a measure of the weight on the bit. That voltage difference iscontinuously applied to the input of converter 16, where the outputsignal 17 is generated, which has a frequency proportional to theD.C.-voltage difference and, thus, also to the weight on the bit.

Simultaneously, as the rotary table (and hence the bit) turns, theabove-described A.C. signal is generated, which completes one cycleevery time the rotary table completes one increment of rotary movement.This signal is introduced into the revolutions channel.

Also, as already described above, output from the revolutions channel isa time gate of sufficient voltage level to switch on the totalizingcounter 5. It has a preselected time duration determined by capacitor 39of the one-shot operational amplifier 38. Such time duration issufficiently long to enable a statistically significant number ofhalf-cycles of signal 17 to be counted.

Using the converter noted above it would be generating outputs of 10 to100 kilohertz for inputs of 10 percent to lOO percent full scale. At thelow end of this range a l-kilohertz signal would have a period of 0.1millisecond. Consequently, a time gate preselected to last millisecondswould count 50 positive half-cycles at the low end of the range and 500positive half-cycles at the high end.

The reason for electing to use every other positive half-cycle of signalB, rather than every positive halfcycle can now be explained. For thesituation described hereinbefore, where there are 150 electrical cyclesper revolution of the rotary table, and for rotary speeds that may be ashigh as 2 revolutions per second, the frequency of signal A and ofsignal B will be 300 per second, and the period will be 3.3milliseconds. If there is to be a time gate of 5 milliseconds initiatedby each positive half-cycle of signal B at a rotary speed of 2revolutions per second, the trigger signals occurring at 3.3-millisecond intervals are too close together to accommodate a5-millisecond time gate. Consequently, it was decided to employ onlyevery other positive half-cycle of signal B. This allows 6.6milliseconds, which is sufficient to accommodate the S-millisecond timegate.

An advantage of this invention is the improved accuracy that can beobtained by the use of combined analog and digital techniques for themeasurement of quantities over extended periods of time. This is incomparison with simple analog methods alone, such as the integration ofa DC. signal. It is well-known that D.C. integration suffers by loss ofsignal over extended periods of time such as to 20 hours, so thataccuracy is poor.

In the present invention analog signals are held in storage for onlybrief intervals, and the conversion of the analog signals tocorresponding pulse trains, or to A.C. signals, together with the use oftime gates to pass these pulses or cycles to a counter at intervalscorresponding to the increments of angular movement of the bit, obviatethe use of DC. integration and enable the use of a digital integrationmethod of high accuracy.

Other modifications and variations of the invention as hereinbefore setforth may be made without departing from the spirit and scope thereof,and therefore only such limitations should be imposed as are indicatedin the appended claims.

I claim:

1. In rotary drilling operations wherein a drill bit is driven by arotary drive engine in continuous rotary movement and a varying amountof weight is placed on the bit, the method of continuously deriving aquantity indicative of the wear on the bit, comprising the steps ofinitiating a signal of preselected and fixed time duration for eachpreselected and fixed increment of rotary movement of the bit,

continuously generating a pulse train having a pulse rate proportionalto the weight on the bit; and counting said pulses only during each ofsaid signal time durations,

whereby the number of pulses registered by the counter during each suchsignal time duration is a measure of the product of the weight on thebit and one increment of rotary movement, and the total number of pulsesregistered by the counter during all such signal time durations is ameasure of the sum of all such products, said sum of products being aquantity indicative of the wear on the bit.

2. The method according to claim 1, wherein said preselected and fixedincrement of rotary movement of the bit is less than one completerevolution.

3. The method according to claim 2, wherein said pulse train comprisesan A.C. signal, of which signal all excursions having a predeterminedsign constitute the pulses of the train.

4. The method according to claim 3, wherein each said signal ofpreselected and fixed time duration is initiated by a time signal froman A.C. tachometer on the rotary drive engine, and said pulse train isderived from a weight transducer connected to the drilling line incombination with a D.C.-to-frequency converter.

5. ln rotary drilling operations having a rotary drive engine, whereinthe drill bit is in continuous rotary movement and a varying amount ofweight is placed on the bit, apparatus for monitoring the wear on thebit comprising in combination means for generating a pulse train havinga pulse rate proportional to the weight on the bit,

means for generating a gate signal having a predetermined time durationonce for each predetermined constant increment of rotary movement on thebit,

a gated pulse counter having two inputs and being adapted for countingsaid pulses during each of said gate signals, and

circuit means for connecting said pulse train means to one of saidcounter inputs and for connecting said gate signal means to the other.

6. The apparatus according to claim 5, wherein said predeterminedconstant increment of rotary movement of the bit is less than onecomplete revolution.

7. The apparatus according to claim 6, wherein said pulse traincomprises an A.C. signal, of which signal all excursions having apredetermined sign constitute the pulses of the train.

8. The apparatus combination according to claim 7, further including anA.C. tachometer on said engine,

and wherein each said gate signal of predetermined time duration isinitiated by a time signal from said A.C. tachometer on the rotary driveengine, and said pulse train is derived from a weight transducerconnected to the drilling line in combination with a DC.- to-frequencyconverter.

9. In rotary drilling operations, wherein a drill bit is maintained incontinuous rotary movement by an engine and said drill bit is suspendedby a drilling line whereby it may be subjected to a varying amount ofweight, and whereby a weight transducer is associated with said drillingline,

the combination including means for monitoring the wear on the bitcomprising means for deriving from said weight transducer connected tothe drilling line an A.C. signal having a frequency of the order of tensof kilohertz and proportional to the weight on the bit,

means for generating a gate signal having a predetermined time durationof at least 5 milliseconds once for each predetermined constantincrement of rotary movement of the bit, said increment being a fractionof one complete revolution, and said gate signal being initiated by atime signal from an A.C. tachometer on the rotary drive engine; and

a gated pulse counter for counting the exursions of said A.C. signalhaving a predetermined sign during each of said gate signals.

1. In rotary drilling operations wherein a drill bit is driven by arotary drive engine in continuous rotary movement and a varying amountof weight is placed on the bit, the method of continuously deriving aquantity indicative of the wear on the bit, comprising the steps ofinitiating a signal of preselected and fixed time duration for eachpreselected and fixed increment of rotary movement of the bit,continuously generating a pulse train having a pulse rate proportionalto the weight on the bit; and counting said pulses only during each ofsaid signal time durations, whereby the number of pulses registered bythe counter during each such signal time duration is a measure of theproduct of the weight on the bit and one increment of rotary movement,and the total number of pulses registered by the counter during all suchsignal time durations is a measure of the sum of all such products, saidsum of products being a quantity indicative of the wear on the bit. 2.The method according to claim 1, wherein said preselected and fixedincrement of rotary movement of the bit is less than one completerevolution.
 3. The method according to claim 2, wherein said pulse traincomprises an A.C. signal, of which signal all excursions having apredetermined sign constitute the pulses of the train.
 4. The methodaccording to claim 3, wherein each said signal of preselected and fixedtime duration is initiated by a time signal from an A.C. tachometer onthe rotary drive engine, and said pulse train is derived from a weighttransducer connected to the drilling line in combination with aD.C.-to-frequency converter.
 5. In rotary drilling operations having arotary drive engine, wherein the drill bit is in continuous rotarymovement and a varying amount of weight is placed on the bit, apparatusfor monitoring the wear on the bit comprising in combination means forgenerating a pulse train having a pulse ratE proportional to the weighton the bit, means for generating a gate signal having a predeterminedtime duration once for each predetermined constant increment of rotarymovement on the bit, a gated pulse counter having two inputs and beingadapted for counting said pulses during each of said gate signals, andcircuit means for connecting said pulse train means to one of saidcounter inputs and for connecting said gate signal means to the other.6. The apparatus according to claim 5, wherein said predeterminedconstant increment of rotary movement of the bit is less than onecomplete revolution.
 7. The apparatus according to claim 6, wherein saidpulse train comprises an A.C. signal, of which signal all excursionshaving a predetermined sign constitute the pulses of the train.
 8. Theapparatus combination according to claim 7, further including an A.C.tachometer on said engine, and wherein each said gate signal ofpredetermined time duration is initiated by a time signal from said A.C.tachometer on the rotary drive engine, and said pulse train is derivedfrom a weight transducer connected to the drilling line in combinationwith a D.C.-to-frequency converter.
 9. In rotary drilling operations,wherein a drill bit is maintained in continuous rotary movement by anengine and said drill bit is suspended by a drilling line whereby it maybe subjected to a varying amount of weight, and whereby a weighttransducer is associated with said drilling line, the combinationincluding means for monitoring the wear on the bit comprising means forderiving from said weight transducer connected to the drilling line anA.C. signal having a frequency of the order of tens of kilohertz andproportional to the weight on the bit, means for generating a gatesignal having a predetermined time duration of at least 5 millisecondsonce for each predetermined constant increment of rotary movement of thebit, said increment being a fraction of one complete revolution, andsaid gate signal being initiated by a time signal from an A.C.tachometer on the rotary drive engine; and a gated pulse counter forcounting the exursions of said A.C. signal having a predetermined signduring each of said gate signals.